This invention relates to a method of synchronizing time in a CDMA system and, more particularly, to a time synchronization method for synchronizing the time of a plurality of base stations and the time of a base station controller that controls these base stations in a CDMA system.
In an IS-95xe2x80x94based N-CDMA (Narrow-band Code Division Multiple Access) system, it is required that signals transmitted from all base stations be synchronized in time (to within xc2x13 xcexcs of absolute time) for the following two reasons:
(1) In an N-CDMA system, a base station is identified by the phase offset of a pilot PN sequence, which is a code string, where PN stands for pseudo-random noise. If base stations are not synchronized in time, the phase offset cannot be stipulated and a terminal such as a cellular telephone will not be able to identify the base station.
More specifically, signals transmitted from each base station to a mobile terminal include a pilot signal and a synchronizing signal, and each base station transmits these signals upon spreading and modulating the signals by a pilot PN sequence. Though the pilot PN sequence code string used is itself the same for each base station, each base station is provided with a different phase offset (a 64-chip unit) so that the mobile terminal can identify the particular base station. (A maximum of 512 base stations can be identified.) The reference for the phase offset is CDMA system time. In regard to a pilot PN sequence for which the phase offset is 0 (zero), the standard is that 15 consecutive xe2x80x9c0xe2x80x9d s followed by a xe2x80x9c1xe2x80x9d be output at time 00:00:00 on Jan. 6, 1980 (the moment at which the changeover from 0 to 1 is made is time 00:00:00). Unless the base stations are synchronized in time, therefore, it is not possible to stipulate the phase offset of the pilot PN sequence and, as a result, it will not be possible for a mobile terminal to identify to which base station it is wirelessly connected.
(2) In order to avoid transmission line congestion and a fluctuation in transmission delay that accompanies such congestion in an N-CDMA system, phase offsets are imposed on the traffic channels (a traffic channel is a channel for voice signals between a mobile terminal and a base station) in steps of 1.25 ms on a per-call basis. If base stations are not synchronized to one another in terms of time, therefore, it will no longer be possible to implement soft handoff between base stations, soft handoff being a characterizing feature of an N-CDMA system. (Soft handoff is the ability of a mobile station to move from one base station to another without an interruption in service.)
More specifically, an example of a signal sent and received between each base station and a mobile terminal is a voice signal transmitted via a traffic channel. In order to avoid the effects of congestion, delay and a fluctuation in delay time in the transmission lines between base stations and a base station controller and between the base station controller and switching equipment or the like, phase offsets are allocated to the voice signals in steps of 1.25 ms per call on each traffic channel. Since a 20-ms frame is partitioned into units of 1.25 ms, in such case there will be 16 offsets.
FIG. 7 is a diagram useful in describing the necessity of a phase offset. Shown in FIG. 1 are a base station controller 1, a base station 2 and mobile terminals 51, 52, . . . , 5n currently communicating with the base station 2. Though data transmission from each terminal is illustrated in the form of bursts in order to make it easier to visualize operation, in actuality the data is transmitted continuously or discretely along the time axis. When voice signals from the terminals 51, 52, . . . , 5n arrive at the base station 2 at the same timing, as shown in FIG. 7, the signals are queued because there is only one transmission line between the base station 2 and the base station controller 1. As a consequence, a certain signal will be sent from the base station 2 to the base station controller 1 late in terms of the numerical order. For example, in terms of the numerical order, a voice signal n is sent late at a timing a in FIG. 7, and a voice signal 1 is sent late at a timing b. If the queuing time and the numerical order are always constant, no problems arise. However, since the terminals move, a slight disparity develops in the order in which the voice signals arrive at the base station 2. When the order of signal arrival differs, the order in which signals are sent from the base station to the base station controller 1 also changes and, as a result, a large variation in transmission delay time is produced. For this reason the 1.25-ms offset is set for each call and only two to three terminals are allocated to one offset to prevent a large fluctuation in transmission time.
Hitless handoff (soft handoff) between base stations under the control of the same base station controller is possible on the condition that the radio frequency before and after handoff is the same and, moreover, that the phase offset allocated to the traffic channels is the same. If the radio frequency is different, an interruption in service will be unavoidable owing to the frequency changeover. If the phase offset is different, this will result in a long standby time at a voice decoder or the like and eventually lead to an interruption in service. It should be noted that one item of voice data should be receivable in 20 ms and that any fluctuation is less than 1.25 ms at most.
Unless the base stations are synchronized in time, therefore, specifying the same offset before and after handoff will be meaningless and hitless soft handoff will be impossible to accomplish.
In view of reasons (1) and (2) set forth above, a first CDMA system according to the prior art is such that a highly accurate GPS receiver (having a time error on the order of xc2x10.1 xcexcs) is deployed at all base stations and the circuitry in each base station is actuated based upon time information and a clock signal received from GPS satellites.
With a second CDMA system according to the prior art, namely the system disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 8-265838, a GPS clock output by a highly accurate GPS receiver deployed at a base station controller is adopted as a master clock and base stations are kept in frequency and time synchronization taking into account the transmission delay time between the base station controller and each base station. According to this second CDMA system, the transmission delay time is measured immediately prior to the sending and receiving of voice signals.
The first CDMA system according to the prior art is disadvantageous owing to the high cost of the base stations and system overall. The high-precision GPS receiver is costly (several hundred thousand yen) and a redundant configuration is necessary in order to reduce base station downtime due to failure. Furthermore, since an inexpensive GPS receiver has a time error on the order of 2 to 3 xcexcs, such a receiver cannot meet the system specifications.
The second CDMA system according to the prior art is capable of maintaining the time and phase synchronization between the base station controller and base stations but a problem that arises is that the base stations themselves cannot achieve time synchronization to absolute time (or to a specific time standard). Further, the second CDMA system is such that transmission delay time is not measured periodically but only just prior to sending/receiving of a voice signal (voice communication). This means that the system cannot deal with a situation in which the transmitting apparatus re-synchronizes for some reason or in which the apparatus recovers after the occurrence of a failure.
Accordingly, an object of the present invention is to provide a time synchronization method through which the time of all base stations can be synchronized to absolute time highly accurately without providing each base station with a GPS receiver, i.e., by a low-cost arrangement.
Another object of the present invention is to provide a time synchronization method through which communication can be performed correctly by synchronizing transmitted signals from all base stations in a highly accurate manner.
A further object of the present invention is to provide a time synchronization method through which the time of all base stations can be synchronized to absolute time, resulting in that a signal to be transmitted can be transmitted upon synchronizing it to a transmitted signal from another base station even if a transmitting apparatus performs re-synchronization for some reason or recovers after the occurrence of a failure.
In accordance with the present invention, the foregoing objects are attained by providing a time synchronization method for synchronizing the time of a plurality of base stations in a CDMA system and the time of a base station controller that controls each of these base stations, comprising the steps of (1) providing the base station controller with a GPS receiver for receiving a signal from GPS satellites and generating a reference time based upon the received signal; (2) adopting the reference time generated by the GPS receiver as the time of the base station controller, generating time reference information based upon the said time and sending the time reference information to each base station; and (3) synchronizing the time of each base station to the time of the base station controller based upon the time reference information received by each base station. In other words, only the base station controller is provided with a GPS receiver, time reference information is sent from the base station controller to each base station and the time of each base station is synchronized to the time of the base station controller. This makes it possible to synchronize the time of all base stations to absolute time highly accurately through an inexpensive arrangement. In addition, since absolute time can be made the same at all base stations, signals to be transmitted from each of the base stations can be transmitted upon being synchronized highly accurately and it is possible to perform communication that does not cause transmission line congestion and a fluctuation in transmission delay that accompanies it.
Further, the base station controller periodically transmits, to each base station as the time reference information, data for specifying the reference time and time correction data for each base station, and the base station uses the reference time specifying data and the time correction data to correct its own time to that of the base station controller. If this arrangement is adopted, the time in the base station can be synchronized momentarily even if the base station re-synchronizes for some reason or recovers after the occurrence of a failure, and the signal to be transmitted can be transmitted upon achieving synchronization with transmitted signals from other base stations.
In this case the base station controller measures the transmission delay time from the base station controller to a base station, based upon the frame timing of a signal transmitted to the base station and the frame timing of the signal received from each base station, creates time correction data proper to each base station, using this transmission delay time and transmits the time correction data to each base station. Each base station then corrects its own time to that of the base station controller using the reference time specifying data and time correction data.
Further, the base station controller measures the transmission delay time from the base station controller to a base station based upon the frame timing of a signal transmitted to the base station and the frame timing of the signal received from the base station, adds this transmission delay time to the reference time to create the time reference information and then transmits the time reference information to each base station. Each base station then synchronizes its own time to that of the base station controller based upon the time reference information that has been received.
Further, the base station controller measures, every n frames, the transmission delay time from the base station controller to a base station based upon the frame timing of a signal transmitted to the base station and the frame timing of the signal received from the base station, adopts a reference time that follows n frames as the reference time specifying data, adopts time obtained by subtracting the transmission delay time from the period of n frames as the time correction data, and transmits this reference time specifying data and time correction data to each base station. Upon elapse of time indicated by the time correction data, each base station makes its own time agree with the reference time indicated by the reference time specifying data.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.